1. Field of the Invention
The present invention relates to a coating technique, and more specifically, to a coating technique for a gas phase synthesized diamond film.
Diamond is the hardest material in the world, has a Vicker's hardness of 10,000 and a high Young's modulus and heat conductivity, and a superior wear resistance and chemical stability, and these superior properties of diamond lead to hopes for various applications thereof as a bulk material or as a coating material. For example, a diamond film could be applied as a wear resistant coating, as a speaker diaphragm, and as a transparent coating for an optical component.
2. Description of the Related Art
The synthesizing of diamond under high temperatures and high pressures, and recently, by a gas phase chemical reaction process (CVD), has long been studied.
For example, a technique for a chemical vapor deposition of a diamond film by a plasma CVD of CH.sub.4 gas diluted with H.sub.2 has been proposed. In this process, due to the plasma in the plasma CVD, the H.sub.2 is excited and activated H atoms are produced, which remove the amorphous carbon and carbon deposits other than diamond, leaving only the diamond, and thus enable the growth of a diamond CVD film.
Nevertheless, a gas phase synthesized diamond film in general has a very low adhesion strength, and although attempts have been made to increase the nuclei forming density of diamond and provide carbides and other intermediate layers, to thus improve the adhesion strength, good results have not been obtained.
The present inventors further developed the DC plasma jet CVD process for a high speed gas phase synthesis of diamond (see Japanese Unexamined Patent Publication No. 64-33096), and proposed a process for forming a plasma sprayed film including diamond by supplying metal and ceramic powder into the plasma during the synthesis of diamond in the DC plasma jet CVD process (Japanese Unexamined Patent Publication No. 2-22471). In this process, an intermediate layer composed of a plasma spraying material and diamond is formed between the plasma sprayed film and the diamond film, and a high adhesion strength is thus obtained.
FIG. 8 shows an apparatus for the production of a coating film formed of a mixed layer of a plasma spraying material and diamond in accordance with the above process.
The anode 1 surrounds the cathode 2, to thereby form a gas passage 3 for the synthesizing gas, etc. Further, a DC power source 4 is connected between the anode 1 and the cathode 2 and a plasma forming gas, such as a mixed gas of H.sub.2 gas and CH.sub.4 gas, is supplied to the gas passage 3 for the synthesis of diamond. When a DC voltage is applied across the anode 1 and the cathode 2, to produce a DC discharge, a plasma is formed. In FIG. 8, a nozzle 5 for supplying the powder is formed at the tip of the anode 1, and a carrier gas 6 including the plasma spraying powder is supplied from the nozzle 5. The plasma spraying powder is melted in the plasma, is deposited on the surface of the substrate 7, and is solidified to plasma a flame sprayed film.
When Ar or another inert gas is supplied to the gas passage 3 and plasma spraying powder is supplied from the nozzle 5, it is possible to form a plasma sprayed film on the substrate. If powder is not supplied from the nozzle 5 but a diamond synthesis gas is supplied to the gas passage 3, then it is possible to form a diamond film on the substrate. Further, if a diamond synthesis gas is supplied to the gas passage 3 and plasma spraying powder is supplied to the nozzle 5, then the plasma spraying and synthesis of the diamond gas phase are conducted simultaneously, and thus it is possible to form a mixed film of a plasma spraying material and diamond on the substrate.
For example, first a plasma sprayed film 8 comprised of the same material as the substrate, or a material having a good affinity therewith, is formed on the substrate 7, a mixed film 9 of the plasma spraying material and diamond is formed thereover, and a diamond gas phase synthesized film 10 then formed as a top film thereover.
In such a laminated structure, even if, for example, the substrate 7 and diamond film 10 have vastly different thermal expansion coefficients, a diamond film 10 can be formed on the substrate 7 with a good adhesion strength.
Nevertheless, in the conventional process explained above with reference to FIG. 8, the following problems arise:
(1) It is difficult to separately control the conditions for the plasma spraying and diamond synthesis. PA1 (2) During the formation of the mixed layer, the plasma spraying powder is melted in a plasma mainly composed of hydrogen, and thus is adversely affected by the hydrogen. PA1 (3) Since a DC arc discharge is used, the electrode material becomes mixed in the film. PA1 (4) Since a DC arc discharge is used, the discharge is not stable. PA1 (5) The speed of the formation of the diamond film is relatively slow and it is difficult to supply fine amounts of the plasma spraying powder in accordance with same.
A solution to these problems is required.